This invention relates generally to a motor control of the type used to transport the carriage in a small calculator operating on DC power. An analog type apparatus applying a tachometer generator for controlling the motion of a printer carrier using a DC motor has been widely used. With digital type detectors such as rotary encoders, linear encoders or the like becoming readily available in recent years, control systems of the digital type have found applications which utilize such digital type detectors. Because of the resultant improved control precision and simple control circuit arrangements, many digital control apparatuses of the phase-locked loop type have been used which detect a signal from a reference oscillator. The oscillator provides a signal dependent on a speed setting, and a phase-difference signal from an encoder is used for directly switching the voltage applied to the motor for speed control using such digital signals.
Such control systems have been widely used on equipment for civilians as the reference oscillator and phase comparator which provide the phase difference signal, are available commercially as integrated circuits. The control system is advantageous in that it has a relatively high control accuracy as the accuracy in measuring speed is determined by the stability of the reference oscillator. The circuit and the motor generate a reduced amount of heat under such control. However, the control system has a disadvantage in the high gain of the control loop, which cannot be readily changed. The result is that the system is liable to oscillate easily and accordingly, is poor in stability. The problem manifests itself especially where the control system is used in an apparatus for controlling the carriage of a serial printer which requires a high speed response since the frictional and inertial loads are designed to be small as compared with the output of the motor which is used. The difficulty is responsible for an oscillation phenomenon which causes irregular dot intervals and poor printing quality in dot impact printers and ink jet printers of the serial type which print in high dot densities for higher printing quality. The above described difficulties could be eliminated by lowering the gain of the control loop, an arrangement which, however, would require complicated circuitry and result in lowered control accuracy.
What is needed is a motor control for a printer carriage which is simple in construction, low in cost and has high control accuracy and stability with regular dot intervals when printing.